William Schlesinger, Duke University, provided an overview of the global carbon cycle to provide background to the discussion of direct and indirect effects on carbon fluxes. The abiotic elements of the carbon cycle include volcanic emissions, carbonate and silicate rock weathering, and air-sea exchange, (see Figure 1–1), although carbon fluxes expand substantially with the addition of biotic processes (see Figure 1–2). For the mid-1990s, Schlesinger noted that the exchange in and out of the terrestrial biota was more than 100 Pg C/yr. Much of this carbon is either respired within the terrestrial biosphere or is emitted by decomposers or fires. A rough balance exists between the natural exchange of terrestrial inputs and outputs of carbon, with only a small amount escaping decomposition and moving into the soil pool.
The human impact on the carbon cycle is twofold. First, humans are converting forests to agricultural lands and thus releasing carbon from forest stands and soil reservoirs, particularly in the tropics. The burning of fossil fuels also serves as a very large source of carbon in the atmosphere. The amount of atmospheric carbon released from fossil fuel emissions is four or five times larger than estimates of carbon emissions from net vegetation destruction. When comparing the sum of these net human-induced atmospheric emissions with estimated carbon sinks, Schlesinger highlighted the residual sink, which illustrates the current incomplete understanding of carbon uptake. The residual sink (previously called the “missing sink”) represents the apparent imbalance in global CO2 accounting:
Atmospheric increase=Fossil fuel emissions+Net emissions from land use−Ocean uptake−Residual sink,
It has been estimated (by difference) at 2.9 Pg C/yr for the 1990s (Houghton, 2003). Robert Watson of the World Bank noted that the residual carbon sink appears to be increasing, up from 1.9 Pg C/yr in the 1980s. Schlesinger proposed several explanations for this residual sink, including carbon accumulation in the undisturbed terrestrial biosphere due to CO2 fertilization. Forests may also be growing back on previously deforested land or changing their distribution on the landscape in a way that replaces low carbon sinks with high carbon sinks. Research has shown that the Earth’s biota have a strong effect on the global carbon cycle through seasonal fluctuations in CO2 (e.g., Keeling and Whorf, 2004; see Figure 1–3). Because the residual sink is thought to originate from terrestrial processes, the net terrestrial flux can be calculated as follows: